That's right. I want no lines on the stave for the RhythmicStaff, but
I do want lines for the Staff context, and no StaffGroup bracket.

The OP didn't say so -- one is not a mind reader ;)

Peter
Have you tried setting 'line-count of StaffSymbol to 0
rather than removing Staff_symbol_engraver?
Trevor

Indeed, adding \override StaffSymbol #'line-count = #0 to the
RythmicStaff's layout block immediately has things looking better -- but
that left a ridiculously tall bar line at the end of the main staff.

I tried taking the StaffGroup out altogether, as you don't need a
StaffGroup bracket, which seems to "work fine" this time.